1.Firstly, create a good
sized room (300x300x300) and place your info_player_start and a light in there.

2.Place a target_laser entity
against one wall of the room. Give it an "angle" key
and value, so it points across the room in the right direction.

3.Using the
"spawnflags" key, you set the color, size, etc of the laser.

4.You can put a func_button
in and give it a "target" of "laser1" and
give the target_laser a "targetname" of "laser1" and
the button will toggle the laser on and off.

5. Compile your map and check
out your laser.

6. You can also target the
lasers at things like func_trains, for moving lasers. This is very easy to do. Just add a
"targetname" to your func_train, and a "target" to the targ_laser
entity. Then the laser will follow the train around..